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I need help! Micro sized white dots on surface of product

2005

I would like to ask for any assistance to anyone that can solve this problem:

I'm an engineer working in a company that manufactures aluminium components via die-casting.

The products are parts for electronic equipments which are roughly 1-inch x 5 mm x 5 mm. After die-casting, one of the surface is then taken for a lapping process. A lapping process is slightly similar to a polishing process.

before lapping, the colour of the surface is dull gray. After lapping, the surface is shiny with a slightly mirror like reflection.

However, when the view of the lapped surface is magnified by a factor of 50, micro sized white dots are apparent on the surface of the final lapping process.

The raw material composition (melted from an ingot): The code is ADC-12:

Cu: 3%
Si: 10.5%
Mg: < 0.3%
Zn: < 1%
Fe: < 0.9%
Mn: < 0.5%
Ni: < 0.5%
Sn: < 0.3%
Al: balance

May I know how does this happen? How to solve the problem?

I need to know how to solve this problem because our customer rejects the product due to the problem.

Yong Tze Shoong
Engineer - Pasir Gudang, Johor, Malaysia.

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Perfectly normal aspect of the microstructure of this alloy.

Why is it a PROBLEM? -- It is certainly a CHARACTERISTIC, but difficult to see how it could be a PROBLEM. But of course if your customer does not understand the situation, he cannot make an informed decision, and realistically you have to work around his uninformed decision.

Perhaps your diecaster can reduce the size scale of the natural effect by either (1) if he uses sodium modifier in the molten aluminium then he should ensure that not too long a time elapses after he adds the sodium before he casts the aluminium, because the modifying effect of sodium fades rapidly in the molten metal; or (2) using strontium modifier instead of sodium because the effectiveness of strontium in the molten metal lasts for very many times longer than the effect of sodium.

A later thought -- maybe your diecaster isn't using any modifier at all, in which case get him to use one.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
2005

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Thank you for you reply, Mr. Bill Reynolds,

Firstly, I would like to know, what are the white dots? Is it a natural process due to oxidization?

From what you have said, It seems that you are thinking among the paths of the grain refining process. Are they visible due to large grains?

Honestly, I think you are right. However, I'm not sure what (around) 0.04mm sized white dots can do to an electronical component.

Regards,

Yong Tze Shoong
- Johor, Malaysia
2005

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2005

The white dots are primary aluminium which, during solidification, separates in small quantity from the liquid before the main part of the liquid solidifies as an intimate mixture of a high-aluminium-low-silicon component and a low-aluminium-high-silicon component. This intimate mixture is called a eutectic -- if you are already familiar with the concenpt of eutectics, good; if not, I'm sorry but we don't have room here for an article about alloy solidification.

The primary aluminium zones show up because they have a different surface texture from the eutectic after polishing, so they reflect light a bit differently. They are not physically separate particles -- they are simply normal zones of the continuous bulk metal, but it happens that you can see them because of the polishing and particular illumination.

Modification is different from grain refining, and gives much improved mechanical properties. Ingots for melting stock usually contain enough titanium and boron for grain refining, and the fast cooling that occurs with diecasting usually forces a fine grain anyhow -- for those reasons I didn't mention it originally.

But perhaps there is after all not enough residual grain refiner in the metal. You can have the diecaster add a grain refiner to the molten aluminium. Just do a trial run first and see if a finer structure results -- if it does, it shows that there was in fact not enough grain refiner in the ingots; if it doesn't make any difference, then you already have the finest grain available, and you'll have to convince your customer that it's all OK and natural for that alloy.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.